GLOBAL WARMING IS SAID TO BE ABOUT CARBON--which the earth giveth and the earth taketh away. How much carbon? The numbers are too huge for ordinary intuition to grasp. So let's cut them down to size.

A century ago a pioneering American family lived off 40 acres and a mule. Today an American family of four can lay claim to 30 acres of the continent, if we imagine land to be evenly divided among us all. How is the allotment used? One acre goes for home, office, factory, road and highway. Six acres are farmland. Eight are range for livestock. Fifteen are grassland, forest, mountain and desert.

The family digs up 24 metric tons of carbon a year, as coal, oil and gas, and releases it into the air as carbon dioxide. Natural processes cycle about 20 times as much in and out of green plants and the oceans. Still, an extra 24 tons might affect climate, if it accumulates.

So how might our family balance its carbon books? It can dig up less, but to break even it must put some back. Twenty-four tons seems like a lot. But spread over 30 acres that's about 6 ounces of carbon per square yard, or a film averaging about two-thousandths of an inch thick.
Grow new trees. The family's pioneer ancestors had to live entirely off the land, so they deforested a lot of land for crops, pasture and fuel. Since about 1920, however, that process has been reversed. The family has returned at least one, and perhaps two, acres of the homestead to forest.
Plow less land. That's what has made room for new trees. Our family has tripled its agricultural productivity in this century. Better railways and highways, and the fossil fuels that power them, have allowed the family to trade 2 acres of inferior farmland in New England for 1 much better acre on the prairies. High-tech agriculture has done the rest. It could do even more.
Buy a tractor. Grandpa got his horsepower from a horse, which required 2 acres of pasture to feed. And the horse emitted copious amounts of methane, a much more potent greenhouse gas than carbon dioxide. As internal combustion engines go, the second stomach of the ruminant quadruped was a whole lot worse for the atmosphere than the engine that replaced it.
Thwart pests. Until 1905 the family used a lot of lumber for railroad ties, which quickly became food for termites. The introduction of creosote preservatives cut this source of demand by two-thirds. The family used to cede one-third of its cropland to insects, rodents and fungi. Chemicals and packaging now protect the crops, preserve the food, shrink the farm and thus expand the forest.

As best these things can be measured, the North American carbon books are now in balance. Carbon dioxide levels downwind of the estate, out in the Atlantic, are lower than upwind, in the Pacific. Carbon-sink skeptics say they don't see enough new trees to account for the drop. But then, global warming skeptics say they don't see enough human carbon emissions to account for rising temperatures. The weight of the evidence indicates both a warming planet and a huge North American carbon sink. The carbon-sink data are, if anything, the more reliable, because they require only direct measurement today, not estimates of conditions a century ago.

If we can't precisely explain where all the carbon is sinking, it's because it's hard to track deposits that average 0.002 inch over a vast continent. Many forest inventories count only "lumber quality" trunks, ignoring younger trees and grassland. New forests mean new, carbon-rich soil, which is almost impossible to inventory accurately. New soil means new sources of silt in rivers, which dump carbon into the ocean. Pampers sequester carbon, too, as long as the landfill they wind up in doesn't permit them to decompose.

Land-poor Europe leveled most of its forests centuries ago, and is preoccupied now with protecting the cow pastures of reactionary farmers. The developing world still depends on 19th-century technologies, which expand humanity's footprint over the surface rather than shrinking it. To save a forest, buy some Third World farmer a tractor. Help him make the same transition we have made, to technologies and fuels that return dustbowl to grassland, pasture to forest, and carbon to earth.

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